Rubber, natural unvulcanised

Rotatory power, determination of, 504 Rubber, natural unvulcanised, 1022 synthetic, 1022 Rubber stoppers, 55 boring of, 56... 

Mention may here be made of the fact that natural unvulcanised rubber is poly-isoprene (2-methylbutadiene) ... 

Polyethylene, polypropylene, natural rubber latex, unvulcanised butyl rubber... 

Tables 5.4 and 5.5 predict that unvulcanised natural rubber (8 = 16.5) will be dissolved in toluene (8 = 18.2) and in carbon tetrachloride (8 = 17.5) but not in ethanol (8 = 26.0), all values being in units ofMPa. This is found to be true. Similarly it is found that there is a wide range of solvents for polystyrene in the solubility parameter range 17.2-19.7 MPa. ...

Like NR, SBR is an unsaturated hydrocarbon polymer. Hence unvulcanised compounds will dissolve in most hydrocarbon solvents and other liquids of similar solubility parameter, whilst vulcanised stocks will swell extensively. Both materials will also undergo many olefinic-type reactions such as oxidation, ozone attack, halogenation, hydrohalogenation and so on, although the activity and detailed reactions differ because of the presence of the adjacent methyl group to the double bond in the natural rubber molecule. Both rubbers may be reinforced by carbon black and neither can be classed as heat-resisting rubbers. 

Unvulcanised natural rubber which has been cooled quickly while under tension. In this condition it behaves like a relatively inextensible fibrous material. 

Vulcanisation of rubbers can be carried out as a separate process after shaping. Shaping can take place at relatively low temperatures since the unvulcanised rubber has an outspoken plastic nature. It is still solid enough to be transferred from the mould to a separate vulcanisation chamber, in which, at a much higher temperature, e.g. by steam, the vulcanisation is achieved. 

Current industrial practice in the rubber industry is for these instruments to measure a property, usually stiffness, changing progressively with time at a set temperature. The temperature may be set so as to equate to typical cure conditions, or may be of an arbitrary nature if used for quality-control testing. The sample of unvulcanised rubber used will have a hot stiffness or viscosity before any crosslinking occurs. This will increase from a minimum value to a... 

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